1. Field of the Invention
The invention is concerned with a multi-range compensated fluid delivery system which is operable over at least two independent pressure ranges. For example, the delivery system is useable on earthworking vehicles such as motorgraders wherein, for example, a front wheel drive may require relatively high pressure operation under some circumstances. The system of the present invention allows such pressures to be applied at the operators command yet protects other work elements in the system from being damaged by excess pressure by providing a safety circuit which reduces the pump output pressure and hence the pressure in the front wheel drive circuit whenever other implements are being driven by the system.
2. Prior Art
Variable displacement pumps are often utilized in current machinery since their flow output can be reduced to a minimum when there is no flow requirement in the fluid delivery circuits which they supply while still maintaining a positive pressure in the circuit with minimum horsepower consumption. For example, in one such system a pump is employed and the pressure level in the fluid delivery circuit is employed to control the displacement of the pump. By utilizing pump output pressure to control the displacement of the pump, system operating pressure is maintained when all the work circuits are inactive with near zero displacement of the pump, as it need only replenish the leakage in the delivery circuit to maintain the system pressure. Under such conditions the forces controlling pump displacement are counterbalanced and a reduction in pressure when a work circuit is operated will cause the pump to increase displacement to meet the increased flow demands. This system, as well as other similar systems are often referred to as pressure compensated delivery systems because the output flow is pressure dependent.
Also, since pump displacement can be varied, such systems are less dependent on engine r.p.m. and offer uniform response in work circuits over wide variation of engine speeds. In addition such systems provide faster response as they can be economically operated at a positive pressure at or near operating pressure.
Dual range pressure dependent variable flow fluid delivery systems are also known. Such systems can operate at two separate and distinct pressure levels with full pressure compensation at both levels. The use of an electric clutch interposed between the power takeoff and the hydraulic pump which provides fluid to hydraulic lift cylinders is also known. The clutch in one such system cannot be actuated unless the gear shift lever is in neutral position whereby the clutch energizing contacts are closed. The use of a directional control valve adapted to a pressure responsive displacement control mechanism as a means of selectively varying the displacement of a fluid pump is also known as is the use of a remotely controlled pilot operated pressure relief valve as a means of reducing the pump operating pressure to a predetermined lower value. Use of a variable displacement pump which can have its flow regulated to maintain a plurality of flow rates with automatic pressure compensation is likewise known. This can be accomplished by introducing a signal pressure on to two separate pistons that are in contact with the swashplate of a pressure compensated pump. The use of a variable displacement pump having means disposed therewithin for sensing discharge pressure and thus causing the pump to vary its displacement also is known to the art as is the use of a control valve which when actuated will cause the pump to work at a lower displacement and when actuated to a greater degree will cause the pump to approach minimum displacement.
While pressure compensated two level pressure operation is known for fluid delivery circuits such systems are not available which include a safely feature which prevents the delivery of pressurized fluid from the pump of the system at a pressure in a higher operating range whenever a work element which will be damaged by pressurized fluid in said higher operating range is connected to the pressure delivery circuit.